Remove from heat and add vanilla and butter; do not stir. Let cool for about 25 minutes, then beat with a wooden spoon until the fudge just begins to thicken. Add chopped nuts and beat for a few more minutes, until it begins to lose its gloss but is not too thick. Pour or spread in a greased pie plate or 8-inch square pan. Score when set and cut into squares when firm.

To test for soft ball stage:Use fresh cold water each time you test the candy. In about 1 cup of cold water, spoon about 1/2 teaspoon of hot candy. Put you hand into the water and push the candy to form a ball. Gently pick the formed ball up (if it will not form a ball, it is not done) - the soft ball will flatten slightly when removed from water.